Several hundred thousand people die suddenly in the United States each year from acute myocardial infarction, and many more suffer from chronic heart problems. A major contributing factor in both acute and chronic heart problems is a reduction in nutrient blood flow to the muscles of the heart resulting from a reduction of blood flow through the coronary blood vessels. The reduction in flow may be caused by deposits of atherosclerotic plaque on the walls of the blood vessel, which causes a narrowing of the lumen or channel of the blood vessel. When the lumen is sufficiently narrowed, the rate of flow of blood may be so diminished that spontaneous formation of a thrombus or clot occurs by a variety of physiologic mechanisms. As is known, once a blood clot has started to develop, it extends within minutes into the surrounding blood, in part because the proteolytic action of thrombin acts on prothrombin normally present, tending to split this into additional thrombin which causes additional clotting. Thus, the presence of atherosclerotic plaque not only reduces the blood flow to the heart muscle which it nourishes, but is a major predisposing factor in coronary thrombosis.
Among the treatments available for the conditions resulting from plaque formations are pharmacological means such as the use of drugs, for example nitroglycerin, for dilating the coronary blood vessels to improve flow. In surgical treatment may be indicated. One of the surgical techniques commonly used is the coronary bypass, in which a substitute blood vessel shunts or bypasses blood around the blockage. The bypass operation is effective but is expensive and subject to substantial risks.
Percutaneous transluminal balloon catheter angioplasty is an alternative form of treatment. This method involves insertion of a deflated balloon into the lumen of an artery partially obstructed by plaque, and inflation of the balloon in order to enlarge the lumen. The lumen remains expanded after removal of the catheter. The major problem with this technique is restenosis of the narrowed vessel by recurrence of the arterial plaque.
Another technique which has recently received a good deal of attention is transluminal laser catheter angioplasty. This treatment involves introduction into the coronary artery of a fiber optic cable the proximal end of which is connected to a laser energy source. The distal end of the fiber optic cable is directed towards the plaque. The laser is pulsed, and the resulting high energy light pulse vaporizes a portion of the plaque. Many problems remain unsolved in laser catheter angioplasty. Locating the plaque requires some means such as a fiber optic scope to see the region towards which the laser pulse will be directed. The interior of the artery must be illuminated, and a clear liquid introduced into the artery to displace opaque blood from the region to be viewed. Even with a fiber optic scope, however, the plaque may be difficult to distinguish from normal arterial walls. When the energy of the laser discharge is directed towards the arterial walls, the walls may undesirably be perforated. Further problems relate to the difficulty in matching the characteristic of lasers and fiber optic cables to the frequency absorption characteristics of various types of plaque, and the by-products of the destruction of the plaque.
Microwave aided balloon angioplasty is described in U.S. Pat. No. 4,643,186 issued Feb. 17, 1987 in the name of Rosen et al. In the arrangement as described by Rosen et al., a catheter including a microwave transmission line terminates at its distal end in an antenna. The antenna is surrounded by a balloon. During angioplasty, the catheter is introduced into a blood vessel or other vas, and the distal end with the balloon and the antenna is manipulated to a point adjacent the plaque. Microwave power is applied to the proximal end of the catheter and flows to the antenna, which radiates the energy to the plaque for heating and thereby softening the plaque. The balloon is expanded against the softened plaque to thereby expand the lumen of the blood vessel. It would be desirable to be able to determine the expanded size of the balloon to thereby determine the diameter to which the lumen of the blood vessel has been expanded. It would be desirable to be able to guide the catheter with a filament. It would be desirable to be able to heat adjacent tissue both conductively and by radiation absorption.